A negative electrode active material such as silicon is expected to provide a higher capacity than carbon materials such as graphite. However, such a negative electrode active material undergoes large volume changes due to charge/discharge. The large volume changes make the contact between the active material and the current collector insufficient or the like, so that the current-collecting capability decreases. This results in degradation of cycle characteristics.
To improve the current-collecting capability, there has been a proposal to mechanically cover the surface of silicon oxide particles with a carbon material (see Patent Document 1). Also, there has been another proposal to form carbon fibers on the surface of an active material core containing a metal or semimetal capable of forming a lithium alloy, for example, by CVD (see Patent Document 2).    Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-373653    Patent Document 2: Japanese Laid-Open Patent Publication No. 2004-349056